A cysteinyl-tRNA synthetase has been purified from Phaseolus aureus (mung bean); selenocysteine can replace cysteine as substrate in the ATP-pyrophosphate exchange reaction. The enzyme will also be purified from selenium accumulator species of Astragalus and its substrate specificity toward cysteine and selenocysteine will be compared. Similar studies with the methionyl-tRNA synthetase from these plants will also be undertaken. The ability of these enzymes from various sources to charge tRNA with the sulfur and selenium isologs will be investigated.